1. Field of the Invention
This invention pertains to reciprocating pumps and compressors, and more particularly to a reciprocating pump or compressor having a self-aligning piston rod and/or piston head.
2. Description of the Related Art
In reciprocating pumps and compressors, such as in a mud pump used in the oil well drilling industry, precise alignment of longitudinal axes of coupled rods and of rods coupled to piston heads minimizes wear in the reciprocating pump or compressor. Misalignment, on the other hand, leads to premature and excessive wear and scarring of piston cylinder liners. In general, misalignment of the longitudinal axes of coupled rods and piston heads limits the run life of the pump or compressor in which the rods and heads are disposed. The problems associated with misalignment of rods in a mud pump are disclosed in U.S. Pat. No. 5,778,759, issued to Johnson.
U.S. Pat. No. 4,719,845, issued to Dugan, discloses a joint for connecting two rods together for reciprocation along aligned longitudinal axes. The joint is designed to compensate for misalignment while transmitting a reciprocating force. The joint is used in a mud pump for pumping drilling fluid or mud downhole for carrying cuttings away from a drill bit. A piston head is connected to a piston rod, which in turn is connected to a crosshead rod. The crosshead rod is connected to a crank, which is driven by a motor to provide reciprocating motion on the crosshead rod, the piston rod and the piston head.
The Dugan '845 patent discloses a joint for connecting the crosshead rod to the piston rod, which comprises a cylindrical cavity formed in one end of the piston rod and a cylindrical plug formed on the associated end of the crosshead rod. The plug is inserted into the cavity, and the plug has a slightly smaller diameter than that of the cavity to allow for slight axial and/or lateral misalignment. A solid, deformable, non-compressible material is located in the cavity. Misalignrment between the crosshead rod and the piston rod is accommodated by deformation of the non-compressible material. A uniform force is thus said to be transmitted across the surface area of the cavity in the piston rod. However, the non-compressible material tends to wear out, and a number of parts must be assembled, making installation difficult.